Amphidiploid Brassica juncea contains conserved progenitor genomes

To perform a detailed study of genome evolution in the natural Brassica amp hidiploid B. juncea, we have constructed two linkage maps based on RFLP (re striction fragment length polymorphism) markers; one generated from a cross between a resynthesized B. juncea (a chromosome doubled interspecific B. r apa x B. nigra hybrid) and a natural B. juncea cultivar, the other from a c ross between two B. juncea cultivars. By using a common cultivar in both cr osses, the two maps could be unambiguously integrated. All loci exhibited d isomic inheritance of parental alleles in the natural x resynthesized cross , showing that B. rapa chromosomes paired exclusively with their A-genome h omologues in B. juncea and that B. nigra chromosomes likewise paired with t heir B-genome homologues. The maps derived from the two crosses were also p erfectly collinear. Furthermore, these maps were collinear with maps of the diploid progenitor species (B. nigra and B. rapa) produced using the same set of RFLP probes. These data indicate that the genome of B. juncea has re mained essentially unchanged since polyploid formation. Our observations ap pear to refute the suggestion that the formation of polyploid genomes is ac companied by rapid change in genome structure.